The present invention relates to an image reconstruction method and an X-ray computed tomography (CT) system, or more particularly, an image reconstruction method based on an iterative reconstruction technique and an X-ray CT apparatus that performs the image reconstruction.
Iterative reconstruction is known as one of techniques for reconstructing an image on the basis of projection data produced by an X-ray CT apparatus or a positron emission tomography (PET) system.
The iterative reconstruction is such that an estimated distribution of coefficients in a system model is sequentially corrected until projection data produced from the estimated distribution of coefficients will agree with projection data produced from actual measurement. The iterative reconstruction may be referred to as statistical reconstruction. Typical examples of the image reconstruction technique include an ordered subsets expectation maximization (OSEM) technique (refer to, for example, Non-patent Document 1).
[Non-Patent Document 1] “Study of a Convergent Subsetized List-mode EM Reconstruction Algorithm” by A. Rahmin, T. J. Ruth, and V. Sossi (Nuclear Science Symposium Conference Record 2004 IEEE, Oct. 2004, Vol. 6, pp. 3978-3982)
According to the foregoing technique, a spatial array of coefficients formed like a rectangular lattice is adopted as a system model. A system matrix for use in reconstructing an image of the system model has a complex structure and includes an enormous amount of data. The calculation of the system matrix is a large load to be imposed on a computer.
As shown in FIG. 15(a), when a stored matrix having been calculated in advance is used, a large memory capacity is required. If a system matrix is, as shown in FIG. 15(b), calculated at every time of image reconstruction, an image reconstruction time gets longer. If the system matrix is simplified in order to shorten the image reconstruction time, image quality deteriorates.